The present invention relates generally to improvements in spread-spectrum transmission systems and in a preferred embodiment the invention is applied to a vehicle location and tracking system.
A number of information bearing channels can share the same medium and approximately the same frequency band and yet be separated at the receiving end with satisfactory interchannel isolation if suitable pseudo-noise (PN) codes are used asynchronously to direct-sequence modulate the channel carriers at a high rate relative to the data rate. This has the effect of spreading the spectrum of the transmitted energy.
At the receiver, the information in each channel is extracted by cross-correlating the incoming composite stream with the code associated with the desired channel. When the clock rate and the epochs of the in-coming and locally-generated codes match, the spread-spectrum energy is collapsed to the relatively narrow, data bandwidth for that channel whilst all the other channel spectra remain spread.
This method enables a particular medium (e.g. a coaxial-cable transmission line) to carry a virtually unlimited number of channels, separation being achieved at the receiving end by code-division multiple access (CDMA). The performance of the scheme in terms of signal-to-noise ratio depends on the relative orthogonality of the codes; that is, on their cross-correlation properties. A unique feature is the smooth degradation of signal-to-noise ratio as more users come into the system compared to the sudden loss of performance which occurs in a conventional frequency division multiple access (FDMA) system once the channel capacity is exceeded.
The capability of a spread-spectrum channel to reject interference from other signals in other channels and from noise is called the process gain. Mathematically, process gain is given as: EQU G.sub.p =10 log.sub.10 B/b (dB) (1)
where
B=bandwidth of spread-spectrum signal PA1 b=data or information bandwidth PA1 T.sub.R =code repetition period PA1 f.sub.c =chip rate
and it is assumed that the spectral line spacing of the PN codes are small enough for the spectra to be considered continuous.
Consider now the case of one transmitter, one receiver and no data. According to equation (1) the process gain is infinite because b.fwdarw.0. The zero-data example might be a ranging system where it is necessary only to locate the code epoch and, knowing the propagation delay, the range to the transmitter may be calculated; range ambiguity can be avoided by making the code repetition period much greater than the propagation delay. In practice the process gain can be very large, but not infinite, and is limited mainly by the extent of the loss of coherence of the carrier at the receiver relative to the receiver local oscillator. If the `coherence time` of the received carrier is .tau. then b.about..tau..sup.-1 and process gain can be increased only by spreading the spectrum of the transmitted signal still further. This can be done by increasing the chip-rate (code clock rate) of the PN code up to a limit set by the electronics or by the ability of the transmission medium to support the spread-spectrum bandwidth
Referring to FIG. 1 it may be seen that in a spread-spectrum location and tracking system, the vehicle 10 or object to be located emits a continual direct sequence spread-spectrum radio signal 11. This transmission is received at a number of well-spaced receiving stations 12 in the coverage area and the differences in the times of arrival of the signals at these receivers are measured. Inverse hyperbolic navigation techniques then may be used to compute the position of the transmitter at the central computer 13 which then sends this information to an operator terminal.
Direct sequence spread-spectrum modulation is employed for a number of reasons, one of which is to minimise multi-path effects. Also, since for location and tracking purposes there is no data transmission requirement, there would appear to be potential for very high process gain. Unfortunately the process gain is severely limited in practice. Firstly transmissions from a vehicle moving in an urban, or suburban, area experience Rayleigh scattering and Doppler frequency-shift. As a result, at each receiving site 12 the received signal spectrum is bandlimited to within .+-..DELTA.f of the centre frequency where .DELTA.f=f.sub.o v/c is the maximum Doppler frequency-shift for a vehicle with speed v transmitting on a frequency f.sub.o (c is the speed of radio propagation). The coherence time of the carrier depends roughly inversely on the width of the frequency-modulation spectrum so that this scattering sets a lower limit to b, the post-correlation bandwidth. Secondly, the radio-frequency spread-spectrum bandwidth cannot be made arbitrarily wide because of limitations on the coherence bandwidth caused by different fading in different parts of the spectrum.
A rough estimate of the available process gain using urban mobile transmitters may be obtained from published data. For a centre frequency of about 450 MHz the minimum coherence time is about 5 ms and the coherence bandwidth is around 1 MHz giving an available process gain of approximately 37 dB. This figure gives a measure of the level of signal enhancement, over broadband spectrally-continuous noise and interference, achievable by receiver processing.
For a spread-spectrum multi-vehicle location and tracking system in which M transmitters are operating simultaneously, each transmitter to be located and tracked has (M-1) interferers. If CDMA is used, the cross-correlation properties of the codes of the wanted and unwanted signals will determine the extent of the interference. In the commonly-used binary Gold code family, the cross-correlation between any pair of codes generated using n-bit shift registers is bounded by EQU .vertline..THETA.(r).ltoreq.2.sup.(n+1)/2 +1 (n odd) EQU .vertline..THETA.(r).ltoreq.2.sup.(n+2)/2 -1 (n even)
Since these sequences are of maximal length, the number of bits in the code is: EQU N=2.sup.n -1
and for n&gt;&gt;1 the ratio of the auto-correlation peak to the maximum cross-correlation bound is EQU R.about.2.sup.(n-1)/2 (n odd) EQU .about.2.sup.(n-2)/2 (n even)
The larger n is made, the better the wanted signal can be distinguished from the unwanted ones. In other words, the longer the sequence length (N) the better. However, EQU N=T.sub.R f.sub.c ( 2)
where
and, as we have seen already, for an urban vehicle-tracking system, both T.sub.R and f.sub.c have practical upper limits set by the coherence time and coherence bandwidth respectively so there is a practical upper limit set on the choice of N. For the particular example quoted above we have N.about.5000 With this value of N we have n.about.12 and hence R.about.32 giving a maximum `process gain` of about 15 dB. Clearly in this case CDMA falls well short when its performance is compared to the available process gain (over an interference continuum) of 37 dB.
It is important to understand that the spectral components of a spread-spectrum signal are spaced by f.sub.R =1/T.sub.R =f.sub.c N. For a given chip rate, long PN codes have spectral lines very close together and short PN codes have widely-separated lines. A long code may be modelled to have a continuous power spectrum but with a short code the discrete lines must be considered, particularly as they affect the process gain which varies in discrete steps according to the number of spectral lines falling into the passband of the post-correlation filter.
The usefulness of a vehicle-tracking or locating system is enhanced in proportion to the number of vehicles which can be located or tracked at the same time. A high, realisable, process gain is needed in such a spread-spectrum multi-vehicle tracking system because of the necessity of isolating each received transmission from the others; a requirement which is exacerbated by the `near-far problem`.
This invention exploits the quasi-discrete nature of the mobile transmitters' spectra and employs a novel form of frequency division multiple access (FDMA) to effect this isolation.